Drill holes can leave sharp edges on the reverse side. Not something you want in your jewelry. One way to remove them is to use a metal file; this works if the metal is hard or you will be doing more fabrication to the piece. What to do if you are finishing up a piece or the metal is soft? Load a pin vise with a size larger drill bit and gently use it like a countersink. Slowly drill from the back side, against the burr. The size larger bit will remove the offending burrs and won't compromise the diameter of the drilled hole.